Intensity modulated direct detection broad optical-spectrum source communication

ABSTRACT

Optical systems and methods for transmission of multiple beams and direct detection of those beams are described. One transmitter for use in a free space optical communication system includes a broad spectrum light source and an optical component including a plurality of sections positioned to receive an optical beam produced by the broad spectrum light source. The sections of the optical component are formed to introduce optical path differences into portions of the optical beam that impinge on the optical component such that each section introduces a delay into a corresponding portion of the optical beam. The introduced delays cause each portion of the optical beam to lack coherence with other portions of the optical beam. A direct detection receiver detects the intensity of the aggregate beams and produces a signal with improved signal-to-noise ratio. The disclosed technology can be used with modulated optical beams such as intensity modulated beams.

TECHNICAL FIELD

The subject matter of this patent document generally relates todetection of optical signals and, more specifically, to a directdetection of multiple optical signals from a source having a broadspectrum.

BACKGROUND

Wireless communication systems transfer data from a transmitter of onestation to a receiver of another station. In some applications, one ofthe stations can be ground based (i.e., stationary) while the otherstation is carried by a flying vehicle (e.g., a satellite in Earth'sorbit, an airplane or an unmanned aerial vehicle (UAV)). Furthermore,multiple stations can be ground based and in communication with one ormore flying objects, both stations can be part of flying vehicles, orboth stations can be ground-based. These wireless communication systemsare sometimes used for Internet connections, especially if theland-based network is underdeveloped. These ground/airbornecommunication systems have to uplink (UL) and downlink (DL) large andever-increasing volumes of data. Such large volumes of data form today'scomplex telecommunication devices and networks, and are fast outpacingbandwidth offered by today's satellite communications technology.Airborne vehicles typically communicate with other airborne orground-based stations using microwave or radiofrequency (RF) bands.However, a major challenge for conventional microwave and RFcommunications is the highly constrained spectrum allocation imposed onthese communication bands.

Free-space optical (laser) communications (FSO or Lasercom) is immune tospectrum allocation due to virtually unlimited bandwidth of the opticalregions (greater than 100 THz). While experimental FSO technology is nowdownlinking data at 10's of Gb/s from air or space, these solutions arecomplex and expensive, require relatively large components, and consumelarge amounts of power. One of the challenges associated with free spaceoptical communications is atmospheric turbulence that interferes withthe optical signals that traverse through large sections of theatmosphere.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram illustrating a free space communicationsystem within which the disclosed technology can be implemented.

FIG. 1B is a simplified diagram of communication links betweenground-based, air-based and space-based stations.

FIG. 2A is a simplified diagram illustrating side view of two Fresnellenses.

FIG. 2B is a simplified diagram illustrating a top view of a Fresnellens.

FIG. 3A is a simplified diagram illustrating an optical beam that isincident on a specially designed structure in accordance with an exampleembodiment.

FIG. 3B is a simplified diagram illustrating an optical beam that isincident on another specially designed structure in accordance with anexample embodiment.

FIG. 4A is a simplified diagram illustrating positioning of opticalstructures within a telescope-like configuration in accordance with anexample embodiment.

FIG. 4B is a simplified diagram illustrating an optical system thatincludes a lens and an optical structure for introducing delays indifferent sections of an optical beam in accordance with an exampleembodiment.

FIG. 4C is a simplified diagram illustrating an optical system thatincludes an optical structure for introducing delays in differentsections of an optical beam in accordance with another exampleembodiment.

FIG. 4D is a simplified diagram illustrating an optical system thatincludes a lens and an optical structure for introducing delays indifferent sections of an optical beam in accordance with another exampleembodiment.

FIG. 4E is a simplified diagram illustrating an optical system thatincludes a lens and an optical structure for introducing delays indifferent sections of an optical beam in accordance with another exampleembodiment.

FIG. 5 is a block diagram illustrating a set of operations that can becarried out to allow transmission of multiple beams in a free spaceoptical communication system in accordance with an example embodiment.

DETAILED DESCRIPTION

In the following description, for purposes of explanation and notlimitation, details and descriptions are set forth in order to provide athorough understanding of the disclosed embodiments. However, it will beapparent to those skilled in the art that the present invention may bepracticed in other embodiments that depart from these details anddescriptions.

The disclosed embodiments facilitate transmission and direct detectionof optical signals in a free space optical communication system using abroad spectrum optical source. One aspect of the disclosed embodimentsrelates to an optical transmitter for transmitting light in a free spaceoptical communication system that includes a broad spectrum lightsource, and an optical component including a plurality of sections thatare positioned to receive an optical beam produced by the broad spectrumlight source. The plurality of sections of the optical components areformed to introduce optical path differences into portions of theoptical beam that impinge on the optical component such that eachsection introduces a delay into a corresponding portion of the opticalbeam that is different from a delay introduced into other portions ofthe optical beam by other sections of the optical component. The delaysintroduced by the plurality of sections cause each portion of theoptical beam that impinges on a corresponding section of the opticalcomponent to lack coherence with other portions of the optical beam thatimpinge on other sections of the optical component.

In some embodiments, the optical beam received by the optical componentis a modulated optical beam having a modulation rate, r, and each of theplurality of the sections of the optical component is formed to producea particular optical path difference that is smaller than c/r, where cis the speed of light in vacuum. For example, the optical pathdifference is at least an order of magnitude smaller than c/r. In someembodiments, each of the plurality of the sections of the opticalcomponent is formed to produce a particular optical path difference thatis larger than c/Δf, where c is the speed of light in vacuum and Δf isthe spectral bandwidth of the broad spectrum light source. For example,optical path difference is at least two orders of magnitude larger thanc/Δf.

In some embodiments, the optical beam received by the optical componentis one of an intensity modulated optical beam. In some embodiments, thebroad spectrum light source is one of: a broad-band laser diode (LD), asuper-luminescent diode (SLD), a light-emitting diode (LED), or anamplified spontaneous emission (ASE) source. In some embodiments, theoptical beam received by the optical component is a modulated opticalbeam having a modulation rate, r, and the light source has a spectralbandwidth that is larger than r. For example, the spectral bandwidth isat least 2 orders of magnitude larger than r. In some embodiments, theoptical component is a Fresnel lens with a plurality of circular zones,and each of the plurality of sections of the optical componentcorresponds to one circular zone of the Fresnel lens.

In some embodiments, the optical component is a plate with a step-likespatial profile including a plurality of segments, and each of theplurality of sections of the optical component corresponds to onesegment of the plurality of segments of the plate. In some embodiments,the broad spectrum light source and the optical component are locatedinside one or both of (a) an unmanned flying vehicle or (b) aground-based station, and the optical component is positioned to providea plurality of the portions of optical beam, after propagation throughthe multiple sections of the optical component, for transmission ontoatmosphere. In some embodiments, the optical component is positioned atthe exit aperture of the optical transmitter.

Another aspect of the disclosed embodiments relates to a free spaceoptical communication system that includes a transmitter and a receiver.The transmitter of the system includes a broad spectrum light source andan optical component that includes a plurality of sections and ispositioned to receive an optical beam produced by the broad spectrumlight source. The plurality of sections of the optical component areformed to introduce optical path differences into portions of theoptical beam that impinge on the optical component such that eachsection introduces a delay into a corresponding portion of the opticalbeam that is different from a delay introduced into other portions ofthe optical beam by other sections of the optical component. The delaysintroduced by the plurality of sections of the optical component alsocause each portion of the optical beam that impinges on a correspondingsection of the optical component to lack coherence with other portionsof the optical beam that impinge on other sections of the opticalcomponent. The receiver of the above described system is positioned toreceive a plurality of the portions of the optical beam that exits theoptical component of the transmitter after propagation throughatmosphere. The receiver includes a photodetector to sense an intensityof the received portions of the optical beam, and enables detection ofthe received optical beam using a direct detection technique.

Turning now to figures, FIG. 1A is a block diagram illustrating a freespace communication system 100 that uses a variety of optical, RF andmm-wave communication links. The communication links in FIG. 1A can beestablished among moving and stationary components, including aircraft102 (e.g., UAVs) and ground-based stations 104, 106. FIG. 1B is asimplified diagram that illustrates communications between a variety ofground-based, air-based and space-based stations. In particular, FIG. 1Bshows ground-to-ground communications between two ground-based stations,such as towers 108 a,108 b, ground-to-air communications between aground-based station 110 a and an airborne station 112 a, air-to-aircommunications between two air-based stations 112 b, 112 c, air-to-spacecommunications between an air-based station 112 d and a space-basedstation 114 a, and ground-to-space communications between a ground-basedstation 110 b and a space-based station 114 b. The disclosed embodimentscan be used to facilitate free-space communications among any of theabove described ground, air or space-based stations.

Detection of optical signals in such systems is carried out using whatis traditionally classified as either a coherent detection technique ora direct detection technique. In coherent detection, informationregarding the phase of the modulated optical signal is used to recoverthe information symbols of the received signal. In direct (alsosometimes called “incoherent”) detection, the phase information isignored and only the intensity of the received optical beam isascertained. As a result, direct detection systems are often simpler andless costly to implement than their coherent counterparts. Suchbenefits, however, come at a cost: coherent systems often outperformdirect detection systems in terms of their immunity to noise andinterference. Intensity modulation (IM) is one class of signalmodulation that is specifically suited for direct detection. In IM,information is encoded into the optical signal as variations in theoptical output power of a source and therefore the detected intensityvariations can lead to the recovery of the modulated signals. Pulsewidth modulation is a subset of intensity modulation. Pulse AmplitudeModulation, On-Off Keying and Pulse Position Modulation are also subsetsof intensity modulation.

One of the challenges in laser communications (“Lasercom”) relates tothe propagation of the optical beams through the atmosphere. Morespecifically, atmospheric turbulence leads to scintillation, causing theamount of light that is received at a receiver to vary in time becauseof atmospheric interference effects. Such variations can be especiallyproblematic for intensity-modulated direct detection techniques thatrely on variations in intensity of the optical beam to recover themodulated information.

One solution to mitigate such adverse effects of atmospheric propagationis to use intensity modulated direct detection with multiple beams. Forexample, multiple transmitters can be implemented that transmitsimultaneous intensity modulated beams (e.g., light beams that areturned on and off or otherwise modulated at the same time) that do nothave phase coherence and thus do not interfere with one another at thereceiver. That is, due to a lack of phase correlation, the receivedoptical signals do not add constructively or destructively, but rathersuch received signals can be treated as non-interfering signals. Each ofthe received beams has a fluctuating power signal, but because thereceived signals are independent or uncorrelated, those fluctuations arereduced when the signals are summed together in the detector. One of theproblems associated with such a multi-beaming technique, however, is thecomplexity of the system, which requires several transmitters to be usedand operated in a synchronized manner to transmit multiple beams,generally all at the same time and aligned to point in the samedirection.

The disclosed embodiments relate to direct detection of intensitymodulated signals that use a single broad spectrum light source yetenable the transmission and/or detection of multiple beams. Thedisclosed technology improves the detection capabilities of a directdetection system while enabling a cost-effective and simple design byeliminating the need for multiple transmitters and associatedcomponents, such as additional modulators, as well as reducing theoperational overhead, such as additional alignment and calibrationprocedures.

In some embodiments, a broad spectrum source is used on the transmitterside of the free space optical communication system, and intentionaltime delays of predetermined durations are introduced in differentsections of the beam. Because the source has a broad spectrum, theintroduced delays remove the coherence effects among the correspondingbeam sections, thus effectively transforming the single source intomultiple transmitters. As such, in some embodiments, a single lightsource and a single modulator is used to effectuate an intensitymodulated direct detection (IMDD) system with multi-beams.

The intentional delays can be introduced using several mechanisms. Onevery convenient way is to use a Fresnel lens, which commonly has a lowcost. FIG. 2A is a lens diagram illustrating side views of an exampleFresnel lens 202 and FIG. 2B shows an exemplary top view of a Fresnellens 206. Fresnel lenses with different thicknesses, different edgeshapes, and different material can be formed to produce the desiredfocusing and diffractive properties of light that passes through thelenses. In the most general sense, the Fresnel lens design allows theconstruction of a lens with a large aperture and a short focal lengthwithout the mass and volume of material that would be required for alens of conventional design. In particular, Fresnel lenses can be mademuch thinner than comparable conventional lenses, and in some casestaking the form of a flat sheet. As evident from FIG. 2A, the opticalpath lengths for a light beam propagates through a Fresnel lens varyacross different zones or annular sections of the Fresnel lens due tovarying thicknesses of those zones or sections. Therefore, each sectionor zone of the Fresnel lens introduces a different delay in acorresponding section of the optical beam that impinges on the Fresnellens.

In some exemplary embodiments, proper amounts of delay are introducedinto the optical beam using a specially designed component other than aFresnel lens. For example, FIG. 3A is a block diagram that illustrates aspecially designed Structure A with a step-like profile with foursections. The sections of the Structure A are characterized as havingdifferent optical path differences, L1 to L4, that introduce differentamounts of delay into the optical beam, such as Optical Beam A from,e.g., a fiber end, which propagates through those sections. FIG. 3Bshows another block diagram that illustrates a specifically designedStructure B with a step profile that introduces similar delays as inFIG. 3A but with a modified spatial arrangement of the step profile ofStructure B. By the way of example, FIG. 3B illustrates a collimatedOptical Beam B that is incident on Structure B. The structures that areshown in FIGS. 3A and 3B can be readily manufactured at a low cost.Other exemplary optical structures with different shapes and profiles,and comprising different materials, can be constructed to providespecific delays into the optical beam.

The light sources for implementing the disclosed systems can have arelatively broad spectrum. In some implementation, the light sourceincludes one or more of the following light sources: a broad-band laserdiode (LD), a super-luminescent diode (SLD), a light-emitting diode(LED), or an amplified spontaneous emission (ASE) source. An SLD istypically an edge-emitting semiconductor light source based onsuperluminescence, which combines the high power and brightness of laserdiodes with the low coherence of conventional light-emitting diodes. Theemission band of an SLD can be 5-100 nm wide.

In some embodiments, when the optical signal is modulated with data at asymbol rate, r, the spectral bandwidth of the light source, Δf, isselected to be much wider than r. That is, Δf>>r. It should be notedthat symbol rate, r, and symbol duration, T_(S), have an inverserelationship: T_(S)=1/r. In implementations where the delays areintroduced into the transmit beam, an optical component, such as aFresnel lens, introduces delays into different sections of the transmitaperture (i.e., the sub-apertures) such that the optical pathdifference, L, introduced in each section follows the followingrelationships: L<<c/r and L>>c/Δf, where c is the speed of light. Thefirst relationship, L<<c/r, or equivalently, L<<c T_(S), ensures thatthe spatial modulation is not impaired by mis-timing and also reducesintersymbol interference. The second relationship, L>>c/Δf, ensures thateach delayed section of the beam is incoherent with other sections ofthe beam. That is, the optical path difference is selected to be muchlarger than the spatial extent of a symbol.

In some example implementations, where a Fresnel lens with N circularzones is used, N different delays are introduced into the optical beam.The total difference in path length between the center and edge zonescan be characterized as:NL _(Shift)=√{square root over (f ² +R ²)}−f  (1).

In Equation (1), L_(Shift) is the optical path difference introduced byeach zone, f is the focal length of the lens, and R is the radius of thelast zone of the Fresnel lens as measured from the center of the lens.For the above example implementations, NL_(Shift)<<cT_(S) to reduceintersymbol interference, and L_(Shift)>>c/Δf so that light that exitseach zone lacks coherence with light exiting other zones of the Fresnellens. As a result, the intensity from each zone adds in the far fieldwithout interference. In such implementations where the above describedFresnel lens is used for transmission of the optical beam, the receiverthat receives the transmitted light in the far field detects the totalenergy of the incident beam, including light from each of the zones ofthe transmit aperture. As noted earlier, due to lack of coherency of thereceived beams, the detected energy of at the receiver is the sum ofenergies from each zone, regardless of their phase differences. Becausethe received optical signals are not coherent, the noise due toatmospheric propagation is reduced in the aggregate signal. Increasingthe number, N, of non-coherent signals produced at the transmitter andreceived at the receiver further improves the signal-to-noise ratio ofthe detected optical signal

FIG. 4A through FIG. 4E show different examples of optical systems thatinclude the disclosed optical components for introducing differentamounts of delay in different sections of an optical beam. FIG. 4Aillustrates a first lens 406 a that is positioned to receive light froma light source 402 a and to produce a collimated light beam that isincident on an optical structure 404 a. The optical structure 404 aintroduces different amounts of delay in different sections of theoptical beam. The optical structure 404 a can, for example, be a platestructure such as any of the example structures shown in FIGS. 3A and3B. The optical beam that exits the optical structure 404 a is incidenton a second lens 408 a and subsequently on a third lens 410 a, whichproduces a collimated output beam. The second lens 408 a and third lens410 a form a telescope structure.

FIG. 4B illustrates a lens 406 b that is positioned to received lightfrom a light source 402 b. The collimated light that exits the lens 406b impinges upon an optical structure 404 b that introduces differentamounts of delay in different sections of the optical beam. The opticalstructure 404 b can, for example, be a plate structure such as any ofthe example structures shown in FIGS. 3A and 3B. In FIG. 4C, the opticalbeam produced by the light source 402 c is received by an opticalstructure 404 c that introduces different amounts of delay in differentsections of the optical beam upon exit from the optical structure 404 c.The optical structure 404 c can, for example, be a Fresnel Lens.

FIG. 4D shows another configuration in which light from a light source402 d is received at an optical structure 404 d that introducesdifferent amounts of delay in different sections of the optical beam.The optical structure 404 d can, for example, be a Fresnel Lens. Thelight that exits the optical structure 404 d is received by lens 406 d,which produces a collimated output beam. In FIG. 4E, the light from alight source 402 e is received at a lens 406 e and is subsequentlyincident on an optical structure 404 e that introduces different amountsof delay in different sections of the optical beam. The opticalstructure 404 e can, for example, be a Fresnel Lens.

In some example embodiments, more than one optical structure can beplaced within the optical transmitter to introduce delays. Suchstructures can produce delays in either a cumulative fashion (e.g., eachstructure produces a certain amount of delay that adds to the opticaldelay produced by other structure(s)) or separately (e.g., eachstructure operates on a different portion of the beam).

FIG. 5 illustrates a set of operations that can be carried out to allowtransmission of multiple beams in a free space optical communicationsystem. At block 502, a broad-spectrum light beam is generated. At block504, the intensity of the light beam is modulated to representinformation to be conveyed through the system. At block 506, themodulated light beam is received at an optical component that comprisesa plurality of sections. At block 508, the modulated light beam isallowed to propagate through the optical component. Each section of theoptical component that is illuminated by the light beam introduces aparticular delay into a portion of the light beam that propagatesthrough such section, and each particular delay introduced by acorresponding section of the optical component causes the portion of thelight beam that exits the corresponding section to lack coherence withother portions of the light beam that exit other sections of the opticalcomponent. At block 510, the output light beam is transmitted from thetransmitter, or the transmit terminal, to a receive terminal through anaberrated medium such as the atmosphere having atmospheric turbulence.The received optical beam has varying amounts of delay that areincoherent with respect to one another, and thus can be combined usingdirect detection of the received light intensities. At block 512, thelight beam is received at a receive terminal and direct detection isperformed to measure the optical intensity of the received light beam.At block 514, the transmitted data is estimated from the measuredintensity.

In some embodiments, the modulated light beam has a modulation rate, r,and an optical path difference associated with each section of theoptical component is smaller than c/r, where c is the speed of light invacuum. For example, optical path difference is at least an order ofmagnitude smaller than c/r. According to some embodiments, an opticalpath difference associated with each section of the optical component islarger than c/Δf, where c is the speed of light and Δf is the spectralbandwidth of a broad spectrum light source that produces the modulatedlight beam. For example, the optical path difference is at least twoorders of magnitude larger than c/Δf. In some embodiments, the modulatedlight beam is one of an intensity modulated light beam. In someexemplary embodiments, where the modulated light beam has a modulationrate, r, and the modulated light beam is produced using a light sourcewith a spectral bandwidth that is larger than r. For example, thespectral bandwidth that is at least an order of magnitude larger than r.

In some embodiments, the optical component is a Fresnel lens with aplurality of circular zones, where each of the plurality of sectionscorresponds to one circular zone of the Fresnel lens. In someembodiments, the optical component is a plate with a step-like spatialprofile that includes a plurality of segments, and each of the pluralityof sections corresponds to one segment of the plurality of segments ofthe plate. In some embodiments, the operations of FIG. 5 further includeproviding a plurality of the portions of optical beam, subsequent topropagation through the multiple sections of the optical component, fortransmission onto atmosphere.

The foregoing description of embodiments has been presented for purposesof illustration and description. The foregoing description is notintended to be exhaustive or to limit embodiments of the presentinvention to the precise form disclosed, and modifications andvariations are possible in light of the above teachings or may beacquired from practice of various embodiments. The embodiments discussedherein were chosen and described in order to explain the principles andthe nature of various embodiments and its practical application toenable one skilled in the art to utilize the present invention invarious embodiments and with various modifications as are suited to theparticular use contemplated. The features of the embodiments describedherein may be combined in all possible combinations of methods, devices,modules and systems, as well as in different sequential orders. Anydisclosed implementation or embodiment may further be combined with anyother disclosed implementation or embodiment.

What is claimed is:
 1. An optical transmitter for transmitting light in a free space optical communication system, comprising: a broad spectrum light source; and an optical component including a plurality of sections and positioned to receive an optical beam produced by the broad spectrum light source, wherein: the plurality of sections are formed to introduce optical path differences into portions of the optical beam that impinge on the optical component such that each section introduces a delay into a corresponding portion of the optical beam that is different from a delay introduced into other portions of the optical beam by other sections of the optical component, the delays introduced by the plurality of sections cause each portion of the optical beam that impinges on a corresponding section of the optical component to lack coherence with other portions of the optical beam that impinge on other sections of the optical component, and the optical beam received by the optical component is a modulated optical beam having a modulation rate, r, the light source has a spectral bandwidth, Δf, that is larger than r, each of the plurality of the sections of the optical component is formed to introduce a particular optical path difference that is larger than c/Δf and is also smaller than c/r, where c is the speed of light in vacuum.
 2. The optical transmitter of claim 1, wherein the optical beam received by the optical component is one of an intensity modulated optical beam.
 3. The optical transmitter of claim 1, wherein the broad spectrum light source is one of: a broad-band laser diode (LD), a super-luminescent diode (SLD), a light-emitting diode (LED), or an amplified spontaneous emission (ASE) source.
 4. The optical transmitter of claim 1, wherein: the optical component is a Fresnel lens with a plurality of circular zones, and each of the plurality of sections corresponds to one circular zone of the Fresnel lens.
 5. The optical transmitter of claim 1, wherein: the optical component is a plate with a step-like spatial profile including a plurality of segments, and each of the plurality of sections corresponds to one segment of the plurality of segments of the plate.
 6. The optical transmitter of claim 1, wherein the broad spectrum light source and the optical component are located inside one or both of (a) an unmanned flying vehicle or (b) a ground-based station, and the optical component is positioned to provide a plurality of the portions of optical beam, after propagation through the multiple sections of the optical component, for transmission onto atmosphere.
 7. The optical transmitter of claim 1, wherein the optical component is positioned at the exit aperture of the optical transmitter.
 8. A method for producing multiple beams for transmission at a free space optical communication system, comprising: receiving a modulated light beam at an optical component that comprises a plurality of sections; and allowing the modulated light beam to propagate through the optical component, wherein: each section of the optical component that is illuminated by the light beam introduces a particular delay into a portion of the light beam that propagates through such section, each particular delay introduced by a corresponding section of the optical component causes the portion of the light beam that exits the corresponding section to lack coherence with other portions of the light beam that exits other sections of the optical component, and the optical beam received by the optical component has a modulation rate, r, and is produced by a light source having a spectral bandwidth, Δf, that is larger than r, each of the plurality of the sections of the optical component is formed to introduce a particular optical path difference that is larger than c/Δf and is also smaller than c/r, where c is the speed of light in vacuum.
 9. The method of claim 8, wherein the modulated light beam is one of an intensity modulated light beam.
 10. The method of claim 8, wherein the optical component is a Fresnel lens with a plurality of circular zones, and each of the plurality of sections corresponds to one circular zone of the Fresnel lens.
 11. The method of claim 8, wherein the optical component is a plate with a step-like spatial profile including a plurality of segments, and each of the plurality of sections corresponds to one segment of the plurality of segments of the plate.
 12. The method of claim 8, further comprising transmitting a plurality of the portions of optical beam, subsequent to propagation through the multiple sections of the optical component, through atmosphere.
 13. A free space optical communication system, comprising: a transmitter, comprising: a broad spectrum light source; and an optical component including a plurality of sections and positioned to receive an optical beam produced by the broad spectrum light source, wherein: the plurality of sections are formed to introduce optical path differences into portions of the optical beam that impinge on the optical component such that each section introduces a delay into a corresponding portion of the optical beam that is different from a delay introduced into other portions of the optical beam by other sections of the optical component, the delays introduced by the plurality of sections cause each portion of the optical beam that impinges on a corresponding section of the optical component to lack coherence with other portions of the optical beam that impinge on other sections of the optical component, the optical beam received by the optical component is a modulated optical beam having a modulation rate, r, the light source has a spectral bandwidth, Δf, that is larger than r, each of the plurality of the sections of the optical component is formed to introduce a particular optical path difference that is larger than c/Δf and is also smaller than c/r, where c is the speed of light in vacuum; and a receiver positioned to receive a plurality of the portions of the optical beam that exits the optical component and propagates through atmosphere, the receiver including a photodetector to sense an intensity of the received portions of the optical beam.
 14. The free space optical communication system of claim 13, wherein the broad spectrum light source is one of: a broad-band laser diode (LD), a super-luminescent diode (SLD), a light-emitting diode (LED), or an amplified spontaneous emission (ASE) source. 